Industrial inspection operations, notably for inspecting materials, use image analysis. This type of analysis is notably applied to the inspection and maintenance of the cladding for fuels of nuclear power stations, these claddings being made of zirconium. Because of the very low limit of solubility of hydrogen in zirconium at ambient temperature, it precipitates with the latter in the form of hydrides which may, in certain circumstances, be harmful for the mechanical strength of the claddings. The quantity of hydrides formed at ambient temperature is directly proportional to the hydrogen content in the material. The image analysis of a cladding sample can be used to quantify the hydrogen content shown by this sample, the image being obtained for example by optical microscopy or by electronic scanning microscopy. Other information relating to the morphology of hydrides, for example their mean size, their proximity or their orientation relative to a known axis is also quantifiable by image analysis.
This method has several advantages over the method of vacuum thermal extraction, because it is a localized and non-destructive analysis for hydrides, which is particularly important on irradiated materials. It is thus not necessary to destroy the samples in order to analyze them. One and the same sample may then undergo several examinations. Notably this makes it possible to make the analysis economically viable by avoiding using too many samples, in the case of awkward operations which require much handling.
However, during the preparation of the samples before images are taken, the latter undergo polishing and a chemical attack. The object of the chemical attack is to reveal the hydrides. The acid mixture preferably dissolves the hydrides which are revealed by optical contrast. Unfortunately, the chemical attack also slightly hollows out the matrix around the hydrides and tends to accentuate their size, particularly in thickness. Unlike an examination by electronic scanning microscopy, in backscattered electrons, where it is possible to observe the hydrides in their true dimension, in optical microscopy a hydride is wider the more virulent the chemical attack. It is therefore difficult to carry out a reliable analysis and notably an effective characterization of the hydrides.